5.4.8 Microsoft bounce: mx not in mta-sts policy

What 5.4.8 means

The recipient domain's MX host is not one that its MTA-STS policy lists, so Exchange Online refuses to deliver to it.

Message text

MX hosts of <domain> failed MTA-STS validation

Common causes of 5.4.8

• Recipient domain's MTA-STS policy does not include the MX host that answered

How to fix 5.4.8

1. Check the recipient domain's MX hosts with MX lookup.
2. Ask the recipient domain's administrator to add the MX host to its MTA-STS policy.

Check your domain

Check the MX records of the recipient domainRuns in your browser; nothing is uploaded.

Get your domain's deliverability score

Have the full bounce message? Paste it into the bounce code lookup and it will match the code and the provider for you. Open the bounce code lookup.

How to read 5.4.8

The second part, 5.4.8, is an enhanced status code (RFC 3463 §2) written class.subject.detail. Class 5 is a Permanent failure (5xx): the command was rejected and the sender should not repeat it unchanged (RFC 5321 §4.2.1; RFC 3463 §2).. Subject 4 is network and routing: a problem reaching or routing to the destination. The detail number refines the subject, and providers may define details of their own, which is why one code can read differently at Gmail, Microsoft and Yahoo.

Do not keep resending the same message: a permanent failure repeats until the cause is fixed. Correct the cause, then send a new message.

Official source: learn.microsoft.com. Last verified 2026-10-20.

Related guides

• How to set up MX records (priorities, null MX, migration)
• MX host does not resolve: causes and how to fix it
• Null MX (RFC 7505): publish it for domains with no mail

Related codes

• 5.4.4 SMTP: unable to route
• 5.4.1 Microsoft: relay access denied
• 5.4.1 Microsoft: recipient rejected
• 5.4.14 Microsoft: routing loop
• 5.4.300 Microsoft: not delivered in time
• 550 5.4.5 Gmail: daily sending limit

FAQ

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